A de novo evolved domain improves the cadmium detoxification capacity of limpet metallothioneins

Metallothioneins (MTs) constitute an important family of metal binding proteins. Mollusk MTs, in particular, have been used as model systems to better understand the evolution of their metal binding features and functional adaptation. In the present study two recombinantly produced MTs, LgiMT1 and LgiMT2, and their de novo evolved γ domain, of the marine limpet Lottia gigantea, were analyzed by electronic spectroscopy and mass spectrometry. Both MT proteins, as well as their γ domains, exhibit a strong binding specificity for Cd(II), but not for Zn(II) or Cu(I). The LgiMTs' γ domain renders an M^II₄(SCys)₁₀ cluster with an increased Cd stoichiometry (binding 4 instead of 3 Cd²⁺ ions), representing a novel structural element in the world of MTs, probably featuring an adamantane 3D structure. This cluster significantly improves the Cd(II)-binding performance of the full length proteins and thus contributes to the particularly high Cd coping capacity observed in free-living limpets.

Exploring the Biological Properties of Zn(II) Bisthiosemicarbazone Helicates

The design of artificial helicoidal molecules derived from metal ions with biological properties is one of the objectives within metallosupramolecular chemistry. Herein, we report three zinc helicates derived from a family of bisthiosemicarbazone ligands with different terminal groups, Zn₂(L^Me)₂∙2H₂O 1, Zn₂(L^Ph)₂∙2H₂O 2 and Zn₂(L^PhNO2)₂3, obtained by an electrochemical methodology. These helicates have been fully characterized by different techniques, including X-ray diffraction. Biological studies of the zinc(II) helicates such as toxicity assays with erythrocytes and interaction studies with proteins and oligonucleotides were performed, demonstrating in all cases low toxicity and an absence of covalent interaction with the proteins and oligonucleotides. The in vitro cytotoxicity of the helicates was tested against MCF-7 (human breast carcinoma), A2780 (human ovarian carcinoma cells), NCI-H460 (human lung carcinoma cells) and MRC-5 (normal human lung fibroblasts), comparing the IC₅₀ values with cisplatin. We will try to demonstrate if the terminal substituent of the ligand precursor exerts any effect in toxicity or in the antitumor activity of the zinc helicates.

Metal-dependent glycosylation in recombinant metallothioneins

We show for the first time glycosylation of recombinant metallothioneins (MTs) produced in E. coli. Interestingly, our results show that the glycosylation level of the recombinant MTs is inversely proportional to the degree of protein structuration, and reflects their different metal preferences.

Synthesis and In Vitro Studies of Photoactivatable Semisquaraine-type Pt(II) Complexes

The synthesis, full characterization, photochemical properties, and cytotoxic activity toward cisplatin-resistant cancer cell lines of new semisquaraine-type Pt(II) complexes are presented. The synthesis of eight semisquaraine-type ligands has been carried out by means of an innovative, straightforward methodology. A thorough structural NMR and X-ray diffraction analysis of the new ligands and complexes has been done. Density functional theory calculations have allowed to assign the trans configuration of the platinum center. Through the structural modification of the ligands, it has been possible to synthesize some complexes, which have turned out to be photoactive at wavelengths that allow their activation in cell cultures and, importantly, two of them show remarkable solubility in biological media. Photodegradation processes have been studied in depth, including the structural identification of photoproducts, thus justifying the changes observed after irradiation. From biological assessment, complexes C7 and C8 have been demonstrated to behave as promising photoactivatable compounds in the assayed cancer cell lines. Upon photoactivation, both complexes are capable of inducing a higher cytotoxic effect on the tested cells compared with nonphotoactivated compounds. Among the observed results, it is remarkable to note that C7 showed a PI > 50 in HeLa cells, and C8 showed a PI > 40 in A2780 cells, being also effective over cisplatin-resistant A2780cis cells (PI = 7 and PI = 4, respectively). The mechanism of action of these complexes has been studied, revealing that these photoactivated platinum complexes would actually present a combined mode of action, a therapeutically potential advantage.

The synthesis, full characterization, photochemical properties, and cytotoxic activity toward cisplatin-resistant cancer cell lines of new semisquaraine-type Pt(II) complexes are presented. Eight semisquaraine-type ligands and their corresponding Pt(II) complexes have been studied. These complexes have turned out to be photoactive at wavelengths that allow their activation in cell cultures. Two of them display remarkable solubility in biological media showing a promising behavior as photoactivatable compounds against several cancer cell lines.

Metal-Specificity Divergence between Metallothioneins of Nerita peloronta (Neritimorpha, Gastropoda) Sets the Starting Point for a Novel Chemical MT Classification Proposal

Metallothioneins' (MTs) biological function has been a matter of debate since their discovery. The importance to categorize these cysteine-rich proteins with high coordinating capacity into a specific group led to numerous classification proposals. We proposed a classification based on their metal-binding abilities, gradually sorting them from those with high selectivity towards Zn/Cd to those that are Cu-specific. However, the study of the NpeMT1 and NpeMT2isoforms of Nerita peloronta, has put a new perspective on this classification. N. peloronta has been chosen as a representative mollusk to elucidate the metal-binding abilities of Neritimorpha MTs, an order without any MTs characterized recently. Both isoforms have been recombinantly synthesized in cultures supplemented with ZnII, CdII, or CuII, and the purified metal-MT complexes have been thoroughly characterized by spectroscopic and spectrometric methods, leading to results that confirmed that Neritimorpha share Cd-selective MTs with Caenogastropoda and Heterobranchia, solving a so far unresolved question. NpeMTs show high coordinating preferences towards divalent metal ions, although one of them (NpeMT1) shares features with the so-called genuine Zn-thioneins, while the other (NpeMT2) exhibits a higher preference for Cd. The dissimilarities between the two isoforms let a window open to a new proposal of chemical MT classification.

Tunicates Illuminate the Enigmatic Evolution of Chordate Metallothioneins by Gene Gains and Losses, Independent Modular Expansions, and Functional Convergences

To investigate novel patterns and processes of protein evolution, we have focused in the metallothioneins (MTs), a singular group of metal-binding, cysteine-rich proteins that, due to their high degree of sequence diversity, still represents a "black hole" in Evolutionary Biology. We have identified and analyzed more than 160 new MTs in nonvertebrate chordates (especially in 37 species of ascidians, 4 thaliaceans, and 3 appendicularians) showing that prototypic tunicate MTs are mono-modular proteins with a pervasive preference for cadmium ions, whereas vertebrate and cephalochordate MTs are bimodular proteins with diverse metal preferences. These structural and functional differences imply a complex evolutionary history of chordate MTs-including de novo emergence of genes and domains, processes of convergent evolution, events of gene gains and losses, and recurrent amplifications of functional domains-that would stand for an unprecedented case in the field of protein evolution.

Modular Evolution and Population Variability of Oikopleura dioica Metallothioneins

Chordate Oikopleura dioica probably is the fastest evolving metazoan reported so far, and thereby, a suitable system in which to explore the limits of evolutionary processes. For this reason, and in order to gain new insights on the evolution of protein modularity, we have investigated the organization, function and evolution of multi-modular metallothionein (MT) proteins in O. dioica. MTs are a heterogeneous group of modular proteins defined by their cysteine (C)-rich domains, which confer the capacity of coordinating different transition metal ions. O. dioica has two MTs, a bi-modular OdiMT1 consisting of two domains (t-12C and 12C), and a multi-modular OdiMT2 with six t-12C/12C repeats. By means of mass spectrometry and spectroscopy of metal-protein complexes, we have shown that the 12C domain is able to autonomously bind four divalent metal ions, although the t-12C/12C pair –as it is found in OdiMT1– is the optimized unit for divalent metal binding. We have also shown a direct relationship between the number of the t-12C/12C repeats and the metal-binding capacity of the MTs, which means a stepwise mode of functional and structural evolution for OdiMT2. Finally, after analyzing four different O. dioica populations worldwide distributed, we have detected several OdiMT2 variants with changes in their number of t-12C/12C domain repeats. This finding reveals that the number of repeats fluctuates between current O. dioica populations, which provides a new perspective on the evolution of domain repeat proteins.

Modularity in Protein Evolution: Modular Organization and De Novo Domain Evolution in Mollusk Metallothioneins

Metallothioneins (MTs) are proteins devoted to the control of metal homeostasis and detoxification, and therefore, MTs have been crucial for the adaptation of the living beings to variable situations of metal bioavailability. The evolution of MTs is, however, not yet fully understood, and to provide new insights into it, we have investigated the MTs in the diverse classes of Mollusks. We have shown that most molluskan MTs are bimodular proteins that combine six domains—α, β1, β2, β3, γ, and δ—in a lineage-specific manner. We have functionally characterized the Neritimorpha β₃β₁ and the Patellogastropoda γβ₁ MTs, demonstrating the metal-binding capacity of the new γ domain. Our results have revealed a modular organization of mollusk MT, whose evolution has been impacted by duplication, loss, and de novo emergence of domains. MTs represent a paradigmatic example of modular evolution probably driven by the structural and functional requirements of metal binding.

Functionalized azobenzene platinum(II) complexes as putative anticancer compounds

The synthesis and characterization of four platinum(II) complexes using azobenzenes conveniently functionalized as ligands has been carried out. The characteristic photochemical behavior of the complexes due to the presence of azobenzene-type ligands and the role of the ligands in the activation of the complexes has been studied. Their promising cytotoxicity observed in HeLa cells prompted us to study the mechanism of action of these complexes as cytostatic agents. The interaction of the compounds with DNA, studied by circular dichroism, revealed a differential activity of the Pt(II) complexes upon irradiation. The intercalation abilities of the complexes as well as their reactivity with common proteins present in the blood stream allows to confirm some of the compounds obtained as good anticancer candidates.

Copper(II) N,N,O-Chelating Complexes as Potential Anticancer Agents

Three novel dinuclear Cu(II) complexes based on a N,N,O-chelating salphen-like ligand scaffold and bearing varying aromatic substituents (−H, −Cl, and −Br) have been synthesized and characterized. The experimental and computational data obtained suggest that all three complexes exist in the dimeric form in the solid state and adopt the same conformation. The mass spectrometry and electron paramagnetic resonance results indicate that the dimeric structure coexists with the monomeric form in solution upon solvent (dimethyl sulfoxide and water) coordination. The three synthesized Cu(II) complexes exhibit high potentiality as ROS generators, with the Cu(II)/Cu(I) redox potential inside the biological redox window, and thus being able to biologically undergo Cu(II)/Cu(I) redox cycling. The formation of ROS is one of the most promising reported cell death mechanisms for metal complexes to offer an inherent selectivity to cancer cells. In vitro cytotoxic studies in two different cancer cell lines (HeLa and MCF7) and in a normal fibroblast cell line show promising selective cytotoxicity for cancer cells (IC₅₀ about 25 μM in HeLa cells, which is in the range of cisplatin and improved with respect to carboplatin), hence placing this N,N,O-chelating salphen-like metallic core as a promising scaffold to be explored in the design of future tailor-made Cu(II) cytotoxic compounds.

Three novel dinuclear Cu(II) complexes based on a N,N,O-chelating salphen-like ligand scaffold and bearing varying aromatic substituents (−H, −Cl, and −Br) have been synthesized and characterized. They three exhibit high potentiality as reactive oxygen species (ROS) generators, with the Cu(II)/Cu(I) redox potential inside the biological redox window. In vitro studies in two different cancer cell lines (HeLa and MCF7) and in a normal fibroblast cell line show promising selective cytotoxicity for cancer cells.

Two Unconventional Metallothioneins in the Apple Snail Pomacea bridgesii Have Lost Their Metal Specificity during Adaptation to Freshwater Habitats

Metallothioneins (MTs) are a diverse group of proteins responsible for the control of metal homeostasis and detoxification. To investigate the impact that environmental conditions might have had on the metal-binding abilities of these proteins, we have characterized the MTs from the apple snail Pomacea bridgesii, a gastropod species belonging to the class of Caenogastropoda with an amphibious lifestyle facing diverse situations of metal bioavailability. P. bridgesii has two structurally divergent MTs, named PbrMT1 and PbrMT2, that are longer than other gastropod MTs due to the presence of extra sequence motifs and metal-binding domains. We have characterized the Zn(II), Cd(II), and Cu(I) binding abilities of these two MTs after their heterologous expression in E. coli. Our results have revealed that despite their structural differences, both MTs share an unspecific metal-binding character, and a great ability to cope with elevated amounts of different metal ions. Our analyses have also revealed slight divergences in their metal-binding features: PbrMT1 shows a more pronounced Zn(II)-thionein character than PbrMT2, while the latter has a stronger Cu(I)-thionein character. The characterization of these two unconventional PbrMTs supports the loss of the metal-binding specificity during the evolution of the MTs of the Ampullariid family, and further suggests an evolutionary link of this loss with the adaptation of these gastropod lineages to metal-poor freshwater habitats.

Pb(ii) binding to the brain specific mammalian metallothionein isoform MT3 and its isolated αMT3 and βMT3 domains

The toxicity of lead, one of the most ubiquitous toxic metals, is well known. Some of its pathological effects are related to its preference for the sulfhydryl groups of proteins. Metallothioneins (MT) are a particular family of metalloproteins characterized by their high Cys content that, among other functions, are linked to the detoxification of heavy metals. In mammals, 4 MT isoforms have been found. The MT3 isoform, also called "neuronal growth inhibitory factor", is mainly synthesized in the brain and contains several structural differences that may contribute to important functional differences between it and other MT isoforms. The abilities of recombinant MT3 and its individual αMT3 and βMT3 fragments to bind Pb(ii) have been investigated here, under different pH conditions, by means of spectroscopy, mass spectrometry and isothermal titration calorimetry. The results obtained show that the binding of Pb(ii) to the intact MT3 protein is relatively unaffected by pH, while the individual domains interact with Pb(ii) in a pH-sensitive manner. The mass spectrometry data reveal the evolution with time of the initially formed Pb-MT complexes. In the case of the full length protein, Pb(ii) remains bound for a long period of time. With the isolated fragments, the lead is eventually released. The Pb-species formed depend on the amount of Pb(ii) present in solution. The thermodynamic data recorded, as measured by ITC, for the replacement of Zn(ii) by Pb(ii) in reactions with Zn-MT3, Zn-αMT3 and Zn-βMT3 are all similar, and in all cases, the displacement of Zn(ii) by Pb(ii) is thermodynamically favorable. Zn-Replete and Pb-replete MT3 have distinctive circular dichroism spectra, suggestive of structural differences with different metallation status.

Metallomics reveals a persisting impact of cadmium on the evolution of metal-selective snail metallothioneins

The tiny contribution of cadmium (Cd) to the composition of the earth's crust contrasts with its high biological significance. We suggest that in gastropod clades, the protein family of metallothioneins (MTs) has evolved to specifically detoxify Cd.

Distinct pathways for zinc metabolism in the terrestrial slug Arion vulgaris

In most organisms, the concentration of free Zn2+ is controlled by metallothioneins (MTs). In contrast, no significant proportions of Zn2+ are bound to MTs in the slug, Arion vulgaris. Instead, this species possesses cytoplasmic low-molecular-weight Zn2+ (LMW Zn) binding compound that divert these metal ions into pathways uncoupled from MT metabolism. Zn2+ is accumulated in the midgut gland calcium cells of Arion vulgaris, where they associate with a low-molecular-weight ligand with an apparent molecular mass of ~ 2,000 Da. Mass spectrometry of the semi-purified LMW Zn binding compound combining an electrospray ion source with a differential mobility analyser coupled to a time-of-flight mass spectrometer revealed the presence of four Zn2+-containing ion signals, which arise from disintegration of one higher MW complex resulting in an ion-mobility diameter of 1.62 nm and a molecular mass of 837 Da. We expect that the novel Zn2+ ion storage pathway may be shared by many other gastropods, and particularly species that possess Cd-selective MT isoforms or variants with only very low affinity to Zn2+.

Goodbye to Silvia Atrian

Mercè Capdevila and Òscar Palacios, longtime collaborators and friends of Silvia Atrian, reflect on Silvia's impact on the field of metallomics, as highlighted in a collection of papers published in Metallomics.

Reactivity of Cu(ii)–, Zn(ii)– and Fe(ii)–thiosemicarbazone complexes with glutathione and metallothionein: from stability to dissociation to transmetallation

Metallothionein and glutathione are key players of the fate of Cu(ii)–/Zn(ii)–/Fe(ii)–thiosemicarbazone anticancer drugs in the cytosol/nucleus.

Squaramide-Based Pt(II) Complexes as Potential Oxygen-Regulated Light-Triggered Photocages

Two new squaramide-based platinum(II) complexes C1 and C2 have been synthesized and fully characterized. Their photoresponse has been assessed and is discussed. A remarkable enhancement in the DNA binding activity has been observed for both complexes, up on irradiation. For C2, the release of Pt(II) provoked by its irradiation has been studied. The response of C2 has been found to be regulated by the presence of oxygen. In vitro cytotoxicity tests show an enhancement in the activity of complex C2 after selective irradiation under hypoxic conditions. Resulting Pt(II) species have been isolated and characterized by various analytical methods establishing this type of squaramido-based complexes as a proof of concept for new Pt(II) photocages.

Copper redox chemistry of plant frataxins

The presence of a conserved cysteine residue in the C-terminal amino acid sequences of plant frataxins differentiates these frataxins from those of other kingdoms and may be key in frataxin assembly and function. We report a full study on the ability of Arabidopsis (AtFH) and Zea mays (ZmFH-1 and ZmFH-2) frataxins to assemble into disulfide-bridged dimers by copper-driven oxidation and to revert to monomers by chemical reduction. We monitored the redox assembly-disassembly process by electrospray ionization mass spectrometry, electrophoresis, UV-Vis spectroscopy, and fluorescence measurements. We conclude that plant frataxins AtFH, ZmFH-1 and ZmFH-2 are oxidized by Cu2+ and exhibit redox cysteine monomer - cystine dimer interexchange. Interestingly, the tendency to interconvert is not the same for each protein. Through yeast phenotypic rescue experiments, we show that plant frataxins are important for plant survival under conditions of excess copper, indicating that these proteins might be involved in copper metabolism.

Metal binding functions of metallothioneins in the slugArion vulgarisdiffer from metal-specific isoforms of terrestrial snails

Arion vulgarisis a European slug with a huge potential for accumulating and detoxifying heavy metals.

Metallothioneins of the urochordate Oikopleura dioica have Cys-rich tandem repeats, large size and cadmium-binding preference

Oikopleura dioica has the longest metallothionein described so far, made of repeats generated by a modular and step-wise evolution.

Analysis of Metal-Binding Features of the Wild Type and Two Domain-Truncated Mutant Variants of Littorina littorea Metallothionein Reveals Its Cd-Specific Character

After the resolution of the 3D structure of the Cd₉-aggregate of the Littorina littorea metallothionein (MT), we report here a detailed analysis of the metal binding capabilities of the wild type MT, LlwtMT, and of two truncated mutants lacking either the N-terminal domain, Lltr2MT, or both the N-terminal domain, plus four extra flanking residues (SSVF), Lltr1MT. The recombinant synthesis and in vitro studies of these three proteins revealed that LlwtMT forms unique M₉-LlwtMT complexes with Zn(II) and Cd(II), while yielding a complex mixture of heteronuclear Zn,Cu-LlwtMT species with Cu(I). As expected, the truncated mutants gave rise to unique M₆-LltrMT complexes and Zn,Cu-LltrMT mixtures of lower stoichiometry with respect to LlwtMT, with the SSVF fragment having an influence on their metal binding performance. Our results also revealed a major specificity, and therefore a better metal-coordinating performance of the three proteins for Cd(II) than for Zn(II), although the analysis of the Zn(II)/Cd(II) displacement reaction clearly demonstrates a lack of any type of cooperativity in Cd(II) binding. Contrarily, the analysis of their Cu(I) binding abilities revealed that every LlMT domain is prone to build Cu₄-aggregates, the whole MT working by modules analogously to, as previously described, certain fungal MTs, like those of C. neoformans and T. mesenterica. It is concluded that the Littorina littorea MT is a Cd-specific protein that (beyond its extended binding capacity through an additional Cd-binding domain) confers to Littorina littorea a particular adaptive advantage in its changeable marine habitat.

Anticancer activity of hydroxy- and sulfonamide-azobenzene platinum(II) complexes in cisplatin-resistant ovarian cancer cells

The syntheses of three platinum(II) complexes bearing sulfonamide- ( (E)-2-(4-methylphenylsulfonamido)-2',6'-difluoroazobenzene, HL1) and hydroxy-azo-2,6-difluorobenzene ((E)-2-((2,6-difluorophenyl)diazenyl)phenol, HL2) bidentate ligands is described. These complexes, [Pt(L1)(DMSO)Cl] (1), [Pt(L2)(DMSO)Cl] (2), and [Pt(L2)₂] (3), were characterized by multinuclear NMR spectroscopy, mass spectrometry, and X-ray crystallography. Despite bearing azobenzene functional groups, none of the three complexes undergo photoisomerization. The anticancer activities of these complexes were evaluated in wild-type (A2780) and cisplatin-resistant (A2780CP70) ovarian cancer cells. All three complexes exhibited IC₅₀ values below 10μM and displayed similar activity in both A2780 and A2780CP70 cell lines, indicating that they are not cross-resistant with cisplatin. The DNA-binding properties of 1-3 were investigated by circular dichroism spectroscopy and by agarose gel electrophoresis. Both studies suggest that 1 and 2 form monofunctional DNA adducts.

Metagenomics analysis reveals a new metallothionein family: Sequence and metal-binding features of new environmental cysteine-rich proteins

Metallothioneins are cysteine-rich proteins, which function as (i) metal carriers in basal cell metabolism and (ii) protective metal chelators in conditions of metal excess. Metallothioneins have been characterized from different eukaryotic model and cultivable species. Presently, they are categorized in 15 families but evolutionary relationships between these metallothionein families remain unresolved. Several cysteine-rich protein encoding genes that conferred Cd-tolerance in Cd-sensitive yeast mutants have previously been isolated from soil eukaryotic metatranscriptomes. They were called CRPs for "cysteine-rich proteins". These proteins, of unknown taxonomic origins, share conserved cysteine motifs and could be considered as metallothioneins. In the present work, we analyzed these CRPs with respect to their amino acid sequence features and their metal-binding abilities towards Cd, Zn and Cu metal ions. Sequence analysis revealed that they share common features with different known metallothionein families, but also exhibit unique specific features. Noticeably, CRPs display two separate cysteine-rich domains which, when expressed separately in yeast, confer Cd-tolerance. The N-terminal domain contains some conserved atypical Cys motifs, such as one CCC and two CXCC ones. Five CRPs were expressed and purified as recombinant proteins and their metal-binding characteristics were studied. All these CRPs chelated Cd(II), Zn(II) and Cu(I), although displaying a better capacity for Zn(II) coordination. All CRPs are able to confer Cd-tolerance, and four of them confer Zn-tolerance in the Zn-sensitive zrc1Δ yeast mutant. We designated these CRPs as environmental metallothioneins belonging to a new formerly undescribed metallothionein family.

The Fungus Tremella mesenterica Encodes the Longest Metallothionein Currently Known: Gene, Protein and Metal Binding Characterization

Fungal Cu-thioneins, and among them, the paradigmatic Neurospora crassa metallothionein (MT) (26 residues), were once considered as the shortest MTs -the ubiquitous, versatile metal-binding proteins- among all organisms, and thus representatives of their primeval forms. Nowadays, fungal MTs of diverse lengths and sequence features are known, following the huge heterogeneity of the Kingdom of Fungi. At the opposite end of N. crassa MT, the recently reported Cryptococcus neoformans CnMT1 and CnMT2 (122 and 186 aa) constitute the longest reported fungal MTs, having been identified as virulence factors of this pathogen. CnMTs are high-capacity Cu-thioneins that appear to be built by tandem amplification of a basic unit, a 7-Cys segment homologous to N. crassa MT. Here, we report the in silico, in vivo and in vitro study of a still longer fungal MT, belonging to Tremella mesenterica (TmMT), a saprophytic ascomycete. The TmMT gene has 10 exons, and it yields a 779-bp mature transcript that encodes a 257 residue-long protein. This MT is also built by repeated fragments, but of variable number of Cys: six units of the 7-Cys building blocks-CXCX₃CSCPPGXCXCAXCP-, two fragments of six Cys, plus three Cys at the N-terminus. TmMT metal binding abilities have been analyzed through the spectrophotometric and spectrometric characterization of its recombinant Zn-, Cd- and Cu-complexes. Results allow it to be unambiguous classified as a Cu-thionein, also of extraordinary coordinating capacity. According to this feature, when the TmMT cDNA is expressed in MT-devoid yeast cells, it is capable of restoring a high Cu tolerance level. Since it is not obvious that T. mesenterica shares the same physiological needs for a high capacity Cu-binding protein with C. neoformans, the existence of this peculiar MT might be better explained on the basis of a possible role in Cu-handling for the Cu-enzymes responsible in lignin degradation pathways.

Does Variation of the Inter-Domain Linker Sequence Modulate the Metal Binding Behaviour of Helix pomatia Cd-Metallothionein?

Snail metallothioneins (MTs) constitute an ideal model to study structure/function relationships in these metal-binding polypeptides. Helix pomatia harbours three MT isoforms: the highly specific CdMT and CuMT, and an unspecific Cd/CuMT, which represent paralogous proteins with extremely different metal binding preferences while sharing high sequence similarity. Preceding work allowed assessing that, although, the Cys residues are responsible for metal ion coordination, metal specificity or preference is achieved by diversification of the amino acids interspersed between them. The metal-specific MT polypeptides fold into unique, energetically-optimized complexes of defined metal content, when binding their cognate metal ions, while they produce a mixture of complexes, none of them representing a clear energy minimum, with non-cognate metal ions. Another critical, and so far mostly unexplored, region is the stretch linking the individual MT domains, each of which represents an independent metal cluster. In this work, we have designed and analyzed two HpCdMT constructs with substituted linker segments, and determined their coordination behavior when exposed to both cognate and non-cognate metal ions. Results unequivocally show that neither length nor composition of the inter-domain linker alter the features of the Zn(II)- and Cd(II)-complexes, but surprisingly that they influence their ability to bind Cu(I), the non-cognate metal ion.

Understanding the 7-Cys module amplification of C. neoformans metallothioneins: how high capacity Cu-binding polypeptides are built to neutralize host nutritional immunity

Cryptococcus neoformans metallothioneins (MTs), CnMT1 and CnMT2, have been identified as essential infectivity and virulence factors of this pathogen. Both MTs are unusually long Cu-thioneins, exhibiting protein architecture and metal-binding abilities compatible with the hypothesis of resulting from three and five tandem repetitions of 7-Cys motives, respectively, each of them folding into Cu5-clusters. Through the study of the Zn(II)- and Cu(I)-binding capabilities of several CnMT1 truncated mutants, we show that a 7-Cys segment of CnMT1 folds into Cu5-species, of additive capacity when joined in tandem. All the obtained Cu-complexes share practically similar architectural features, if judging by their almost equivalent CD fingerprints, and they also share their capacity to restore copper tolerance in MT-devoid yeast cells. Besides the analysis of the modular composition of these long fungal MTs, we evaluate the features of the Cys-rich stretch spacer and flanking sequences that allow the construction of stable metal clusters by adjacent union of binding modules. Overall, our data support a mechanism by which some microbial MTs may have evolved to enlarge their original metal co-ordination capacity under the specific selective pressure of counteracting the Cu-based immunity mechanisms evolved by the infected hosts.

Hints for metal-preference protein sequence determinants: different metal binding features of the five tetrahymena thermophila metallothioneins

The metal binding preference of metallothioneins (MTs) groups them in two extreme subsets, the Zn/Cd- and the Cu-thioneins. Ciliates harbor the largest MT gene/protein family reported so far, including 5 paralogs that exhibit relatively low sequence similarity, excepting MTT2 and MTT4. In Tetrahymena thermophila, three MTs (MTT1, MTT3 and MTT5) were considered Cd-thioneins and two (MTT2 and MTT4) Cu-thioneins, according to gene expression inducibility and phylogenetic analysis. In this study, the metal-binding abilities of the five MTT proteins were characterized, to obtain information about the folding and stability of their cognate- and non-cognate metal complexes, and to characterize the T. thermophila MT system at protein level. Hence, the five MTTs were recombinantly synthesized as Zn²⁺-, Cd²⁺- or Cu⁺-complexes, which were analyzed by electrospray mass spectrometry (ESI-MS), circular dichroism (CD), and UV-vis spectrophotometry. Among the Cd-thioneins, MTT1 and MTT5 were optimal for Cd²⁺ coordination, yielding unique Cd₁₇- and Cd₈- complexes, respectively. When binding Zn²⁺, they rendered a mixture of Zn-species. Only MTT5 was capable to coordinate Cu⁺, although yielding heteronuclear Zn-, Cu-species or highly unstable Cu-homometallic species. MTT3 exhibited poor binding abilities both for Cd²⁺ and for Cu⁺, and although not optimally, it yielded the best result when coordinating Zn²⁺. The two Cu-thioneins, MTT2 and MTT4 isoforms formed homometallic Cu-complexes (major Cu₂₀-MTT) upon synthesis in Cu-supplemented hosts. Contrarily, they were unable to fold into stable Cd-complexes, while Zn-MTT species were only recovered for MTT4 (major Zn₁₀-MTT4). Thus, the metal binding preferences of the five T. thermophila MTs correlate well with their previous classification as Cd- and Cu-thioneins, and globally, they can be classified from Zn/Cd- to Cu-thioneins according to the gradation: MTT1>MTT5>MTT3>MTT4>MTT2. The main mechanisms underlying the evolution and specialization of the MTT metal binding preferences may have been internal tandem duplications, presence of doublet and triplet Cys patterns in Zn/Cd-thioneins, and optimization of site specific amino acid determinants (Lys for Zn/Cd- and Asn for Cu-coordination).

Rhenium and technetium tricarbonyl, {M(CO)3} (+) (M = Tc, Re), binding to mammalian metallothioneins: new insights into chemical and radiopharmaceutical implications

This paper deals with the binding of the four mammalian metallothioneins (MTs) to the organometallic metal fragment {fac-M(CO)3}(+) (M = (99)Tc, Re), which is highly promising for the preparation of second-generation radiopharmaceuticals. The study of the transmetallation reaction between zinc and rhenium in Zn7-MT1 by means of UV-vis and CD spectroscopy demonstrated the incorporation of the {fac-Re(CO)3}(+) fragment to the MTs. This reaction should be performed at 70 °C to accelerate the reaction rate, a result that is consistent with the reported reactivity of the rhenium fragment. ESI-TOF MS demonstrated the formation of mixed-metal species as Zn6,{Re(CO)3}-MT, Zn6,{Re(CO)3}2-MT, and Zn5,{Re(CO)3}3-MT, as well as the different reactivity of the four MT isoforms. Hence, Zn-MT3 showed the highest reactivity, in agreement with its high Cu-thionein character, whereas Zn-MT2 exhibited the lowest reactivity, in line with its high Zn-thionein character. The reactivity of the Zn-loaded forms of MT1 and MT4 is intermediate between those of MT3 and MT2. The study of the binding of the {fac-(99)Tc(CO)3}(+) fragment to MTs showed a significant and very interesting different reactivity in relation to rhenium. The transmetallation reaction is much more effective with technetium than with rhenium and significant amounts of mixed Zn x ,{(99)Tc(CO)3} y -MT species were formed with the four MT isoforms whereas only MT3 rendered similar amounts of rhenium derivatives. The results obtained in this study support the possible use of technetium for labelling mammalian metallothioneins and also for possible radiopharmaceutical applications.

Full characterization of the Cu-, Zn-, and Cd-binding properties of CnMT1 and CnMT2, two metallothioneins of the pathogenic fungus Cryptococcus neoformans acting as virulence factors

We report here the full characterization of the metal binding abilities of CnMT1 and CnMT2, two Cryptococcus neoformans proteins recently identified as metallothioneins (MTs), which have been shown to play a crucial role in the virulence and pathogenicity of this human-infecting fungus. In this work, we first performed a thorough in silico study of the CnMT1 and CnMT2 genes, cDNAs and corresponding encoded products. Subsequently, the Zn(II)-, Cd(II)- and Cu(I) binding abilities of both proteins were fully determined through the analysis of the metal-to-protein stoichiometries and the structural features (determined by ESI-MS, CD, ICP-AES and UV-vis spectroscopies) of the corresponding recombinant Zn-, Cd- and Cu-MT preparations synthesized in metal-enriched media. Finally, the analysis of the Zn/Cd and Zn/Cu replacement processes of the respective Zn-MT complexes when allowed to react with Cd(II) or Cu(I) aqueous solutions was performed. Comprehensive consideration of all gathered results allows us to consider both isoforms as genuine copper-thioneins, and led to the identification of unprecedented Cu5-core clusters in MTs. CnMT1 and CnMT2 polypeptides appear to be evolutionarily related to the small fungal MTs, probably by ancient tandem-duplication events responding to a highly selective pressure to chelate copper, and far from the properties of Zn- and Cd-thioneins. Finally, we propose a modular structure of the Cu-CnMT1 and Cu-CnMT2 complexes on the basis of Cu5 clusters, concordantly with the modular structure of the sequence of CnMT1 and CnMT2, constituted by three and five Cys-rich units, respectively.

Understanding the interaction of an antitumoral platinum(II) 7-azaindolate complex with proteins and DNA

The reactivity of the [Pt(dmba)(aza-N1)(dmso)] complex 1, (a potential antitumoral drug with lower IC50 than cisplatin in several tumoral cell lines) with different proteins and oligonucleotides is investigated by means of mass spectrometry (ESI-TOF MS). The results obtained show a particular binding behaviour of this platinum(II) complex. The interaction of 1 with the assayed proteins apparently takes place by Pt-binding to the most accessible coordinating amino acids, presumably at the surface of the protein -this avoiding protein denaturation or degradation- with the subsequent release of one or two ligands of 1. The specific reactivity of 1 with distinct proteins allows to conclude that the substituted initial ligand (dmso or azaindolate) is indicative of the nature of the protein donor atom finally bound to the platinum(II) centre, i.e. N- or S-donor amino acid. Molecular modeling calculations suggest that the release of the azaindolate ligand is promoted by a proton transfer to the non-coordinating N present in the azaindolate ring, while the release of the dmso ligand is mainly favoured by the binding of a deprotonated Cys. The interaction of complex 1 with DNA takes always place through the release of the azaindolate ligand. Interestingly, the interaction of 1 with DNA only proceeds when the oligonucleotides are annealed forming a double strand. Complex 1 is also capable to displace ethidium bromide from DNA and it also weakly binds to DNA at the minor groove, as shown by Hoechst 33258 displacement experiments. Furthermore, complex 1 is also a good inhibitor of cathepsin B (an enzyme implicated in a number of cancer related events). Therefore, although compound 1 is definitely able to bind proteins that can hamper its arrival to the nuclear target, it should be taken into consideration as a putative anticancer drug due to its strong interaction with oligonucleotides and its effective inhibition of cat B.

Cryptococcus neoformans copper detoxification machinery is critical for fungal virulence

Copper (Cu) is an essential metal that is toxic at high concentrations. Thus, pathogens often rely on host Cu for growth, but host cells can hyperaccumulate Cu to exert antimicrobial effects. The human fungal pathogen Cryptococcus neoformans encodes many Cu-responsive genes, but their role in infection is unclear. We determined that pulmonary C. neoformans infection results in Cu-specific induction of genes encoding the Cu-detoxifying metallothionein (Cmt) proteins. Mutant strains lacking CMTs or expressing Cmt variants defective in Cu-coordination exhibit severely attenuated virulence and reduced pulmonary colonization. Consistent with the upregulation of Cmt proteins, C. neoformans pulmonary infection results in increased serum Cu concentrations and increases and decreases alveolar macrophage expression of the Cu importer (Ctr1) and ATP7A, a transporter implicated in phagosomal Cu compartmentalization, respectively. These studies indicate that the host mobilizes Cu as an innate antifungal defense but C. neoformans senses and neutralizes toxic Cu to promote infection.

Studying the interactions of a platinum(II) 9-aminoacridine complex with proteins and oligonucleotides by ESI-TOF MS

The interaction of a novel Pt complex, [Pt(dmba)(N9-9AA)(PPh(3))](+)1 (dmba = N,N-dimethylbenzylamine-κN,κC; 9AA = 9-aminoacridine), which exhibits anti-tumor activity, with certain key proteins has been monitored by ESI-MS. Also, the interaction of 1 with a designed double-stranded oligonucleotide containing the GG motif has been followed by mass spectrometry as well as by fluorimetry. The results obtained show the low interaction of 1 with the considered proteins and the absence of covalent interaction with the oligonucleotides, but the fluorimetric data confirm the π-π interaction of 1 with the double-stranded DNA, which is probably the reason of the previously reported activity of 1 in several tumor cell lines.